The present invention relates to a recording reproducing optical system by which at least one of recording and reproducing of the information is conducted onto an optical information recording medium, objective lens, aberration correcting optical element, optical pick-up apparatus and recording reproducing apparatus.
Recently, the investigation and development of a new high density optical disk system in which a blue violet semiconductor laser light source of about 400 nm wavelength, and an objective lens whose numerical aperture (NA) is heightened to about 0.85, are used, is advanced. As an one example, in an optical disk of NA 0.85 and using wavelength 400 nm (hereinafter, in the present specification, called “high density DVD”), in the same dimension as the DVD (NA 0.6, using wavelength 650 nm, storage capacity 4.7 GB), the information of 20-30 CB can be recorded.
As a high NA objective lens appropriate for the high density DVD system, the objective lens of two lenses in two groups composition disclosed in Japanese Tokkaihei No. 10-123410 is publicly known. This objective lens is a lens in which, by dividing the refractive force of the optical surface to the ray of light of the high NA into 4 surfaces, the production error sensitivity of each lens is decreased, however, there is a problem that the production cost is increased by the amount in which a process is necessary by which each lens is assembled. From the view point of the production cost, also in such a high density DVD system, as in the CD system or DVD system, it is desired that the objective lens is structured by one lens in one group composition. Then, for the more cost reduction, it is more desirable that the objective lens for the high density DVD system is a plastic lens.
As the plastic objective lens appropriate for the high density DVD system, one of inventors of the present invention proposed, together with another inventor, an objective lens as described in Japanese Tokkai No. 2001-324673. However, in the plastic lens of NA of about 0.85, when the environmental temperature changes, the change of the wave front aberration which is generated due to the change of the refractive index of the plastic is large, it is a problem in the practical use. In FIG. 1, as an example, the situation of the wave front aberration change of the plastic lens of one lens in one group composition whose NA is 0.85, focal distance is 2.2 mm, and designed reference wavelength is 405 nm to the temperature change, is shown. From FIG. 1, in the plastic lens of NA of 0.85, because the wave front aberration exceeds the Marechal's criterion at the temperature change of ±15° C., it can not be used for the objective lens of the optical disk player in the state as it is.
Accordingly, in the case where high NA plastic objective lens is used in the optical disk player, it is necessary that, by any means, the change of the wave front aberration is corrected. One of the present inventors proposes, as an optical pick-up apparatus for correcting the change of the wave front aberration due to the temperature change of the high NA plastic objective lens, the optical pick-up apparatus as described in Japanese Tokkai No. 2002-08228.
However, in this optical pick-up apparatus, because it is necessary that the aberration correction means for correcting the change of the wave front aberration is operated while the change of the wave front aberration due to the temperature change is dynamically detected by an optical detector, by an amount in which the means for dynamically detecting the change of the wave front aberration due to the temperature change, and the means for operating an aberration correcting means corresponding to the detection result are necessary, the production cost of the optical pick-up apparatus is increased.